The present invention relates to the field of transmyocardial revascularization (TMR), and more particularly, to an improved device and method for optically guiding the process of laser ablation in creating the revascularization channels. Channels are ablated to a proper depth and safety is enhanced by preventing complete perforation of the heart wall, where a fiber-optic or other waveguide can be precisely positioned adjacent an area to be laser-ablated, including at positions adjacent the posterior epicardial or endocardial surfaces of the heart and at trans-septal positions within the chambers of the heart.
Transmyocardial laser revascularization (TMR) is a procedure whereby laser channels are created in the walls of the heart to reduce angina in patients with ischemic coronary artery disease. This procedure is generally used with patients in whom no other revascularization procedure is possible or as an adjunct to other revascularization procedures involving repair or replacement of the epicardial coronary arteries, such as coronary artery bypass surgery or catheter-based angioplasty procedures.
Laser channels are created in the ischemic zones, initially by using direct exposure of the heart through cardiac surgery procedures. Once the heart is exposed, a laser energy delivery device is then placed against the outer surface of the heart and a channel ablated through the heart wall to the inner blood chamber of the heart. This surgical procedure has been effective in clinical trials; however, a non-surgical or less invasive procedure is desired.
Laser catheters were developed so that laser energy could be delivered from inside the left ventricle by passing one or more optical fiber-based devices from a femoral artery cannulation to the left ventricle, usually with fluoroscopic guidance. Catheter-based myocardial revascularization suffers from the possibility of epicardium perforation and consequent uncontrollable bleeding, potentially resulting in cardiac tamponade or coronary artery perforation, causing death in some cases.
Devices such as the NOGA Cardiac Navigation System (Biosense Webster, Inc., Diamond Bar, Calif.) have been used to help guide the laser catheter to the desired location in the left ventricle of the heart. The NOGA device uses a location sensor and electrodes incorporated into a catheter. The device acquires and records intra-cardiac electrical activation and ventricular motion in real-time for each acquired point, the data being used to provide a 3-D dynamic reconstruction of the heart chamber during a cardiac cycle. While useful to spatially locate the distal laser tip within the heart, devices of this sort provide no gauge of the depth of the laser channel created or of the proximity of the laser catheter to the epicardium. Channel depth and proximity to the epicardium are highly predictive of a possible and unwanted perforation. In a recently reported study, the Biosense device was used to guide percutaneous catheter based placement of laser channels. Because there was a fear of perforation of the myocardium and attendant serious complications, and no means of gauging either the channel depth or the distance of the tip of the laser catheter to the outside wall of the heart-the epicardium, clinical investigators were only allowed to use two pulses of laser energy. While this strategy proved to be safe, two laser pulses are not enough to create laser channels effectively. The results of the study showed that the therapy was safe, but there was no therapeutic effect compared to placebo. In fact, two laser pulses only make small indentations on the inside of the heart; no transmyocardial channels (the therapeutic goal) are created.
U.S. Pat. No. 5,893,848 to Negus et al. describes a gauging system using either a mechanical force transducer or an optical or ultrasound pulse emitter and transducer to detect time differences between reflections from the terminus of a channel and from the heart walls. The embodiment of the Negus device utilizing an optical sensor device senses the propagatable ablation energy as it is reflected by a tissue boundary. This embodiment requires an optical ablating energy source, e.g., laser or ultrasound, to measure distance. The potential problem of this sound-wave device in use is that bubbles, produced as an obligate consequence of vaporization of myocardium in creating the laser ablation, will interfere with the ultrasound signal. Ultrasound is highly reflected by gas; thus, the base of the laser channel will be obscured by bubbles during real-time image or signal acquisition. This device is therefore an ineffective means by which to monitor the progress of laser ablation.
While guided laser channels are potentially superior to laser channels being directed by fluoroscopy, clinical trials of the former have shown perforations.
Secondly, it is not known how deep the channels should be and there is currently no effective manner to determine the depth of the channel aside from fluoroscopy.
Randomized clinical trials of open surgical transmyocardial laser revascularization have consistently shown statistically significant efficacious results in reducing patients"" angina and this technology is now FDA-approved. Catheter based trials of laser revascularization have failed to show significant improvement in patient angina. The only prominent difference between the two therapies (aside from a thoracotomy, which has not been shown to reduce angina) has been that the catheter-based therapies do not create transmyocardial channels. Rather, they produce a shallow channel, always less that 5 mm in depth and in most cases 2 mm or less. Assuming that the therapeutic effect is conferred by the channels created, shallow channels appear to be sub-therapeutic.
Channel depths created by percutaneous catheter approaches have been limited, due to safety concerns and the absolute need to avoid perforating the myocardium. In an open-chest surgical procedure, channels that bleed can be monitored and/or sutured shut. Conversely, a left ventricular perforation occurring during a percutaneous catheter-based procedure can cause uncontrolled bleeding and rapid onset of cardiac tamponade which can progress rapidly to shock and death. The only therapy is to place a needle in the chest and remove the pericardial blood which then allows effective cardiac function, but bleeding can continue and may require open-heart surgery.
Accordingly, a need remains for a better way to gauge and control myocardial revascularization.
The present invention overcomes these problems and limitations of the prior art by combining myocardial revascularization with optical reflectance.
Optical coherence reflectance (OCR) (or optical coherence tomography, OCT) is a structure-imaging method using detection of light reflectance signals. Heretofore, the technique has been used primarily to image blood vessels and structures of the eye. OCT uses infrared light to acquire cross-sectional images of tissue on the micrometer scale. OCT uses low-coherence interferometry to produce a two- or three-dimensional image of optical scattering from internal tissue microstructures in a way that is analogous to ultrasonic pulse-echo imaging. The images are taken using near-infrared light, avoiding the dangers associated with ionizing radiation, as with x-ray images.
Near-infrared light penetrates deeply into tissue, making it useful for imaging of internal structure. The majority of the transmitted light is highly scattered as it penetrates into the tissue. Scattered photons dominate in most imaging applications, leading to blurred images. By using a white light Michelson interferometer as a gate, OCT detects only the unscattered photons, thus generating high-resolution images. Further, heterodyning techniques are used to detect very low levels of reflected light from tissue. OCR can detect reflected signals as small as approximately 1xc3x9710xe2x88x929 of the incident optical power.
Because the frequency or wavelength of light is so much shorter than that of ultrasound-mediated imaging technologies, the resolution can be an order of magnitude better. Increased resolution is important, given the scale of the tissues involved in the heart. The resolution of ultrasound is at best approximately 100 microns, whereas OCR can achieve resolutions of approximately 5 microns. This increase translates into increased safety when the operator or surgeon is trying to precisely position a catheter close to the epicardium or avoid perforation while ablating or removing myocardial tissue.
OCT is described in further detail in Optical Coherence Tomography of Ocular Diseases, (C. A. Puliafito et al., SLACK, Inc.: 1996), incorporated by reference. See in particular chapter 1, pp. 3-8 and Appendix, pp. 369-374).
Because OCR data and OCT images are based on the optical backscattering properties of tissue, changes in tissue composition or structure are detectable using this technique.
One aspect of the present invention is a myocardial revascularization device, comprising a catheter suitable for use in a myocardial revascularization procedure, the catheter comprising an elongate body having a proximal end and a distal end; tissue-ablating means housed at least partially within the catheter and capable of effectuating tissue ablation adjacent the distal end of the catheter; and an optical reflectance fiber housed within the catheter, said fiber coupled to an optical reflectance system. The device is operative during a myocardial revascularization treatment to gauge tissue ablation by transmitting a reflectance signal in the reflectance fiber, the reflectance signal comprising either short optical pulses or low-coherence light. The device thereby gauges tissue ablation by means of time-domain reflectance or optical coherence reflectance.
In a preferred embodiment according to the present invention, the tissue-ablating means is laser energy transmitted by an optic fiber, housed at least partially within the catheter, to a tissue ablation site adjacent the distal end of the catheter.
Another aspect of the present invention is a method of measuring a thickness of a candidate tissue during a percutaneous myocardial revascularization procedure using a myocardial revascularization catheter, comprising positioning a distal end of the catheter adjacent the candidate tissue; and measuring the thickness of the candidate tissue using either of time-domain reflectance or optical coherence interferometry.
Another embodiment of the present invention is a method of determining a position of a myocardial revascularization catheter relative to a boundary of a subject tissue in a myocardial revascularization procedure, comprising positioning a distal end of the myocardial revascularization catheter adjacent to the subject tissue; ablating tissue; and locating the distal end of the catheter relative to the boundary of the subject tissue using either of time-domain reflectance or optical coherence interferometry.
The invention will become more readily apparent from the following Detailed Description, which proceeds with reference to the drawings, in which: